A built-in type spindle motor for driving the spindle of a machine tool or the like has been increasingly used recently, to meet the demand for cost reductions, floor space reductions, and miniaturization. The stator of a built-in motor for such a purpose must be accurately fixed to the spindle head housing or the like of a machine tool, and a stator core is built up by superposing high-permeability steel laminations and fixedly uniting the superposed laminations by welding the circumference thereof, and then the stator core is finished to an accurate external size by grinding.
As the laminations of the stator core are welded only in the circumference of the stator core, however, the laminations are not fastened together in the inside portion of the stator core, and thus the inside portion of the stator core has a low rigidity but the outside portion of the stator core has a high rigidity. Therefore, it is possible that the stator core is deformed with a lapse of time when the stator core having such a construction and held by a mandrel is ground and is removed from the mandrel after grinding. It is also possible for the stator core to be deformed during transportation or when subjected to a winding process or a varnish-impregnating process.